Both example links are dead, but that doesn't matter anyway, since it is only a limited set of user's posts visible here.

What would be much more important to have is a search results download, as searching on /b/breaking by "writer" allows looking through all of the user's posts. (e.g.: https://arca.live/b/breaking?target=nickname&keyword=김샤브샤브).

Within the posts, there may be video embeds. Sometimes they are originally gifs or webps. Never met actual mp4's myself, but you have to replace the filetype with gif or webp and set query string to ?type=orig. Not sure if there's a way to know which filetype it actually is - guess you have to just try each.
For example: https://arca.live/b/arknights/122263340 - has both a gif and a webp files.

There doesn't seem to be an api, so an extractor would have to parse html in this case.